[Pw_forum] regarding structural optimization

[konda]

Dear all
I am doing test calculations for orthorhombic  layered compounds having
van der waals forces.  i calculated the volume by varying c/a, b/a and a
with energy and taken the minimum values of c/a, b/a and a. But my
volume change is surprisingly 2% with experimental ( even van der waals
forces having). I tried with the "vc-relax" for same system i found 12%
volume change with experimental.I tried with psedopotentials also and
the results
   are same.
   I am very thankful any to solve this problem.
   Thanks in advance
   S.Appalakondaiah
   Research Scholar
   University of hyderabad
   Hyderabad
   India